1. Field of the Invention
This invention relates to a fire suppression system and, more particularly, to a method and apparatus for suppressing and extinguishing conveyor fires in an underground coal mine.
2. Description of the Related Art
Coal is mined because it will burn and, therefore, is a major source of economical energy. Unfortunately fires occur in underground coal mines. These fires are a threat to the lives of miners. Fires cause great financial losses to the mining companies. According to the Mining Safety and Health Administration, there were 271 reportable fires in underground coal mines in the United States during the ten year period from Jan. 1, 1978 through Dec. 31, 1987. Fortunately most of these fires were handled promptly so the losses were small, but some were not.
Burning coal can be effectively and quickly controlled with water applied as a fire hose stream. If the fire is not discovered quickly or if the fire fighting was not started in good time, this may not work. Fire hose streams have limited range in the restricted height of a mine. The fire almost always takes the course that the ventilation follows. It can only be approached from the upwind side. The downwind side of a fire that has spread to the coal is full of smoke and very hot. The heat of a mine fire also weakens the mine roof, which may cause the roof to cave.
All underground mines (not just coal mines) must include circulating fresh air so that the miners can work. The mining of coal almost always releases methane gas from the coal seam. Breaking the coal from the seam, which really is the mining operation, may release combustible methane gas. Air movement or ventilation is required to dilute the methane to a safe noncombustible concentration and carry the methane-air mix to the surface.
The coal mine is ventilated with “intake air” through a network of “entries” or tunnels in the coal seam. When the intake air reaches the “face” where the actual mining takes place and dilutes and carries away the methane, the air is then called “return air”. Return air is directed into “returns” or “return entries” which conduct the return air to the surface.
The return air is not allowed to contact equipment that might ignite a methane-air mix. The only equipment allowed in the face area and in a return must be “permissible” which means that it has been tested and found to be flameproof. Equipment that will be used only in intake entries does not have to be permissible, but much of the electrical equipment will be dusttight.
It should be apparent that in mining or developing entries, there must be at least two entries, an intake and a return, which must have ventilation. Also, in the production area or “section” where the actual mining is taking place there must be connections for the air to flow between the intakes and the returns. These connecting entries are called “crosscuts”.
Crosscuts are usually spaced about every 100 feet. When the entries have advanced about 100 feet beyond a crosscut, the next crosscut will be started. When this crosscut is finished, the one behind will be closed to ventilation, eventually with a concrete block or metal wall called a “stopping”. In actual practice, there are almost always three or more entries in a section, usually about 100 feet apart, driven with crosscuts between entries.
Modern coal mines can be classified by the mining system used to extract the coal. Virtually all coal mines use powerful “continuous miners” to drive openings in the coal seam.
These machines break the solid coal into pieces so that it can be hauled and/or conveyed to the surface. The openings made are the entries and crosscuts referred to above. Two of the most used mining systems are the longwall mining system and the block or room and pillar system.
Development, or the driving of entries and crosscuts, for the longwall system usually requires driving four, three, or sometimes only two entries with the necessary crosscuts between these entries. Since the entries have different functions and ventilation, usually all of the crosscuts between the entries will be sealed with stoppings to prevent the ventilation air currents from flowing from one entry to its adjacent entry, unless this is needed. These stoppings are effective barriers to a fire in one of the entries and will prevent the fire from spreading to the adjacent entry, at least for a reasonable time.
In contrast, the block system involves driving a larger number of parallel entries, often eight or ten and sometimes even more. With this multiplicity of entries, many will remain connected through open crosscuts and will share the ventilation. A fire in one of the connected entries will tend to stay in the originating entry as it will follow the ventilation, but with the expansion of the hot gases that have gone through the fire zone, the heat and smoke can move into adjacent open entries. Usually it is just a matter of time until the active fire will spread to adjacent entries, unless steps are taken to prevent this spread.
The statistics of fires in underground coal mines are obviously inadequate as only fires that have existed for thirty minutes or more must be reported. Many reports of fires which did burn well over thirty minutes describe scenarios of the fire getting beyond the range of the fire hose streams.
Prior to 1960 virtually all of these fires had to be sealed. Sealing is very expensive as the mine must remain sealed, often for four to six months, to allow the heat of the fire to cool and avoid rekindling when ventilation is restored in the fire area.
In a 1960 fire the new tool of high expansion foam, conceived in Great Britain and developed and tested in the United States, controlled a fire that had spread over an area so that 4100 lineal feet of entries and crosscuts were burning. The foam equipment operated almost continuously for three days. This fire probably was the largest mine fire that was ever controlled by direct fire fighting.
Unfortunately only about sixty of the large high expansion foam generators similar to the one used on the above described fire have been purchased by the coal industry. They are not inexpensive as the current price for the generators, which must be tailor made for the mine, is in the order of thirty to forty-five thousand dollars. Many smaller and less refined generators are commercially available. Based on the fire reports available in the United States, these smaller and less expensive machines have never been successful in controlling a fire that spread to the coal. The result has been that some fires were not fought successfully using this advanced firefighting technique because the foam equipment was unavailable, inadequate, or mishandled.
It is also important to consider the manner in which nonreportable fires have been controlled and also why these techniques have failed in other instances. Because quick response to a fire is so important, virtually all fires that are extinguished are fought by persons who were at or in the vicinity of the fire when it started. If extinguishers are used quickly enough, they can extinguish the fire while it is still small. When the fire spreads to the coal, a fire hose stream can extinguish the fire if it has not spread beyond the range of the hose streams. The rate at which the fire will spread to the coal is greatly affected by the size of the starting fire, the height of the coal seam, the presence of a coal top or “roof”, and the percent volatile of the coal.
Fires seldom get out of control in low seam mines, which usually have noncombustible roof. If the volatile content of the coal is low, the coal will not ignite readily and the rate a fire will spread is slow. These conditions which slow the spread of the fire provide more time to assemble the personnel and equipment needed to fight the fire. The advance of the firefighters is usually fast enough so that they can overtake the leading edge of the fire and extinguish it.
In contrast, the opposite will occur in coal mines having thick seams with high volatile coal, especially at the mine roof. The coal ignites readily, and the fire grows and moves downwind rapidly. This fire situation must be handled very quickly and competently with fire hoses before it gets beyond the range of a hose stream.
Since the maximum range of a fire hose adjusted to a solid stream in high coal is in the order of 40 to 50 feet, the fire will be large before it gets away from the fire hose streams. Because the fire is large and active, the downwind side of the fire is very hot with dense smoke. In the past attempts have been made to set a fire hose in a crosscut on the downwind side of the fire close to the entry to stop the fire, but the published fire reports indicate that these attempts have not been successful in difficult fire situations.
It is important to understand the conditions that make it difficult to fight fires in a coal mine. The amount of air that reaches the upwind side of the fire is the amount needed to provide safe conditions for the firefighters who are using the fire hoses. Usually, the ventilation air velocity must be in the order of 200 to 300 feet per minute (fpm) to prevent the smoke from the fire from moving back against this fresh air. Higher velocity will feed the fire and cause it to grow rapidly. Air velocity of 200 to 300 fpm is sufficient to feed a large fire. Accordingly, several devices for detecting and suppressing underground fires have been developed.
U.S. Pat. No. 3,684,021 discloses apparatus for detecting and suppressing a potentially dangerous flame from moving away from a mine face and down a mine tunnel. A flame extinguishing agent is not discharged upon the occurrence of ignition of small pockets of either methane or coal dust adjacent the mine face. As a rule most of the flames ignited adjacent the mine face terminate quickly or are extinguished by water from spray equipment mounted on the mining machine for suppressing dust and maintaining the cutter bits cool during the material dislodging operation.
U.S. Pat. No. 3,684,021 discloses that once a flame progresses from the mine face and advances in the entry where it begins to be fueled by ventilation air, the advancing flame is detected. A flame suppressing agent is then discharged ahead of the advancing flame. The agent completely fills the cross-sectional opening of the entry. When the flame reaches the area of the entry filled with the extinguishing agent, the flame is quenched and prevented from igniting an explosive mixture of airborne coal dust and/or methane gas.
U.S. Pat. No. 924,599 also discloses a method of extinguishing fires in a mine by sealing off the advancing flame. Upon the occurrence of a fire, the entrance to a heading is sealed by closing a door and a network of pipes are opened to the entry. An exhaust pump is connected to the network of pipes so that air is drawn out of the entry into the pipes. Consequently the atmospheric pressure in the sealed entry is reduced. The air normally supporting combustion is withdrawn and the fire is extinguished. The network of pipes can also be used to introduce steam to the entry to contribute to the fire suppression.
While it has been proposed to attach a nozzle or spray head to the outlet end of fire hose or pipe and advance the pipe outlet into a confined structure to flood the structure with water to suppress the flame, the known devices, as disclosed in U.S. Pat. No. 2,747,933, are not adaptable for use in fighting fires in an underground mine. It is also known to mount pipes or conduits on wheels to move the conduits to a desired position where water is sprayed from nozzles spaced along the length of the mobile pipe. These types of devices are most commonly utilized in lawn sprinkling systems as disclosed in U.S. Pat. Nos. 1,191,643; 1,282,142; 1,368,269; and 3,807,635.
U.S. Pat. No. 2,769,664 discloses a mobile sprinkler-type irrigation system. This system permits water to be conveyed through a network of pipes to a distance point where water is dispersed in a spray pattern from spaced-apart nozzles. The systems are not adaptable for use in an underground mine to suppress a fire.
U.S. Pat. No. 5,909,777 discloses a water pipe assembly that delivers water under pressure through nozzles to generate a series of intersecting sprays directed at selected angles. The water spray prevents the roof bolts from being heated to an elevated temperature which can cause a loss of anchorage of the bolts in the mine roof. The water curtain also cools the hot gases generated by the fire to stop advance of the fire beyond the curtain so that the fire can be contained and extinguished.
Fire suppression systems for conveyors in underground mines, such as coal mines, are of particular importance. Conveyors require automatic fire suppression systems that meet the requirements set forth by the relevant Mine Safety and Heath Administration regulations. Existing conveyor fire suppression systems typically include conventional fastening systems that attach to various points on or around the conveyor. These systems often fail to meet regulations and may be difficult to install. Violations can occur when systems fail to properly position water discharge devices, fail to include functional piping arrangements, and lack structural integrity. In many cases, skilled fire suppression installers are not available for system installations.
Prepackaged systems provide ease of installation, but do not guarantee the proper orientation of the sprinkler head. With the sprinkler head in an incorrect position, the orifices are susceptible to being clogged by sediment found in the water system and/or water is applied in an inefficient manner.
Other prepackaged systems guarantee the proper positioning of the sprinkler but are cumbersome to install. These systems require detailed information to be analyzed to design a compliant system. Accordingly, there is a need to provide an improved conveyor fire suppression system.